Explosion of Oil Field Sump Tank

An abrupt, catastrophic explosion and rupture of a sump tank occurred. The sump tank was actually not a "normal" sump tank, but was a two compartment tank. A solid, full section, plate of steel separated both tank sections and there was no passageway for gases or fluids to pass from one compartment to the other. The subject sump tank was partially buried. The two tanks were welded together, giving a visual appearance of being a single tank. Operating personnel would have had absolutely no indication that the subject tank was, in fact, two completely independent tanks. The tank has been described as a short tank (which contained sump liquid) and a long tank (which contained a non-functional pressure gauge). The two tanks were separated by a solid baffle which prevented gaseous or liquid communication between the long and short tank sections.

Evidently, the workmen were attempting to pressurize the larger tank section in order to "blowout" or displace sump tank fluids from the smaller tank section into a nearby waste storage vessel.

It was reported that the pressure source being used was gas from the high pressure gas sales line. It was further reported that the gas pressure being used by the workmen at the time of the rupture was probably about 800 psi. The failed long tank, sitting on a two wheel dolly, is shown in Photograph A.

Photograph A: Overall profile of the long tank.
The failed circumferential weld is located
 at the right end of the tank as presented
 in this photograph (see white arrows).

The circumferential weld which failed is shown to the right in Photograph A (see white arrows). A "rear" quarter view, showing an overall view of the failed end of the long tank is shown in Photograph B.

Photograph B:  Rear quarter view of the
 long tank, showing the failed circumferential
 weld (see red arrows).

A side profile view of the "short" tank is shown in Photograph C.

Photograph C: Profile view of the short tank.
The circumferential weld which failed (and which
 attached the long tank to the short tank) is shown
 at the left in this photograph (see red arrows).

The circumferential weld which failed is at the left end of the short tank in Photograph C (see red arrows). Photograph D shows the baffle plate which completely separated the "short" tank from the "long" tank.

Photograph D: Overall view of the failed weld
 and baffle plate on the short tank.

It is noted that the baffle shown in Photograph D is slightly dished, i.e., concaved. It is evident that this "baffle" did not allow liquid or gaseous communication between the two tanks.

The long tank fracture surface and the mating short tank fracture surface were photographed in detail. Visual examination of the girth weld fracture surface revealed three significant fracture features. An overall view of the short tank fracture surface is shown in Photograph E.

Photograph E: Overall view of a section of
 the failed circumferential weld on the short
tank. Red arrows denote location of
welding slag in root of circumferential weld.

The root of the circumferential weld contains what appears to be welding slag and evidence of incomplete weld root penetration. The welding slag is the darker, gray material at the weld root located at the junction between the weld metal and the outside diameter of the baffle (see red arrows in photo E). Additionally, above the incomplete root penetration area (toward the outside weld surface), a darker discolored thickness of weld metal is observed. This appears to be a region where the weld failed progressively at some prior time, under the influence of cyclic pressure, by metal fatigue. The fact that this area of the fracture is darker colored (more corroded), is evidence of crack growth occurring over a period of time, as a result of periodic or cyclic pressure.

A third area of fracture, created during the tank rupture is located near the outside edge of the failed weld (see white arrows). A close-up view of the three distinct areas within the short tank fracture is shown in Photograph F. The area exhibiting lack of root penetration is denoted with red arrows.

Photograph F: Close-up view of details of
 circumferential weld fracture. Red arrows
 denote slag and lack of weld root penetration.
 The white bracket denotes an area of old crack
 growth. The out (lighter colored), more recent
 fracture is denoted with white arrows.

The area of the fracture exhibiting "old fatigue fracture" is denoted with a white bracket in Photograph F. The area of newer, fresher fracture is denoted with white arrows.

When critical pressure levels occurred, the crack increased in size. As a result, the thickness of weld metal available to resist these pressurization stresses diminished.

On the day of the incident, the pressure which workmen put into the long tank was of sufficient magnitude that the effective wall thickness that remained in the girth weld was insufficient to resist the resulting longitudinal stress and the vessel rupture occurred.

As a result of the studies conducted in this matter, it was determined that:

1. Failure of the subject sump tank was the result of pressurization of the long tank.

2. The girth or circumferential weld connecting the long and short tanks had been weakened by fatigue cracking and/or cyclic tearing resulting from prior pressurization during the vessel's lifetime.

3. The subject long/short tank girth weld exhibited lack of weld root penetration. This lack of weld root penetration provided a possible source for metal fatigue and/or cyclic tearing crack initiation.

4. The vessel, as configured at the time of the accident, did not have a pressure relief valve. Such a relief valve would have limited the user's ability to apply pressures sufficient to result in crack growth within the subject girth weld. Such a relief valve would have reduced or eliminated the possibility of catastrophic vessel rupture.

5. The pressure gauge on the subject tank was visually inoperable and thus could not be relied upon to indicate the tank's internal pressure.

6. The tank owner had not verified that the subject tank was suitable for use as a sump tank, i.e., by pressurizing one side of the tank, sump fluids should be forced from the other side. That is, the tank owner had not verified that pressure communication existed between the long and short tanks.

7. The tank owner had not verified the integrity of the sump tank for its intended purpose. The tank owner had apparently not determined the integrity of the circumferential girth weld.

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